1. Field of Invention
This invention relates to electromagnetic flowmeters energized by tri-state excitation of a rectangular waveform; and, more particularly, to such electromagnetic flowmeters which are substantially free of noise.
2. Description of Prior Art
The trend of developement of electromagnetic flowmeters has been from a conventional excitation system utilizing a commercially available frequency, toward excitation systems utilizing low-frequency rectangular waveforms with good stability at a zero point. The rectangular waveform excitation systems include bi-state excitation systems wherein the flowmeter is excited with two values, a positive value and a negative value; and tri-state excitation systems wherein the flowmeter is excited with the above two values (i.e. a positive value and a negative value) and a zero excitation period before and after the two values. These excitation systems produce different types of noise between the electrodes of a detection system disposed in the flowmeter. The noises are removed by various arithmetic systems which have advantages and disadvantages.
Known signal processing systems for removing noise produced in the tri-state excitation systems are disclosed, for example, in Japanese Laid-open Patent Publication Nos. 53-75966 and 57-149919. However, these prior systems do not take into account the fact the magnetic core used in the excitation system or the magnetic casing used in the flowmeter, such as for passing the fluid, is usually used as a return path for magnetic flux applied to the fluid to be measured. More specifically, the core or casing is magnetic and hence exhibits magnetic hysteresis. Thus, a residual magnetic field remains applied by the core or casing to the fluid, even during quiescent periods during which excitation current is no longer applied to the excitation system.
Stated otherwise, an electromotive force, responsible for error, is generated due to the residual magnetic field and the flow speed of the fluid during the quiescent period when no electric current is applied. Such electromotive force has different polarities dependent on the hysteresis even if the flow rate of the fluid is constant, since the residual magnetic field has opposite polarities in the quiescent period wherein the exciting current falls from a positive mode to zero, and in a quiescent period wherein the exciting current falls from a negative mode to zero. No highly accurate detection of flow rates is therefore possible with the conventional noise removing system wherein no provision is made for noise resulting from the magnetic hysteresis of various flowmeter components.